1. Field of the Invention
The present invention relates to a process for efficiently separating methanol from a mixture of methyl acrylate or methyl methacrylate with methanol. It also relates to a process for efficiently separating methanol from a mixture of methyl acrylate or methyl methacrylate with methanol and water. Hereafter, the term (meth)acrylic acid may be used to refer to either acrylic acid or methacrylic acid and the term methyl (meth)acrylate refers to either methyl acrylate or methyl methacrylate.
2. Description of the Related Art
It is known in the art that esterification of (meth)acrylic acid with methanol or ester exchange of an alcohol with methyl (meth)acrylate yields a mixture of methanol, methyl (meth)acrylate, and water.
The esterification reaction, which normally calls for an excess molar amount of methanol with respect to (meth)acrylic acid, essentially consumes all of the (meth)acrylic acid leaving no unreacted (meth)acrylic acid. Therefore, the liquid reaction mixture after the reaction is a liquid mixture comprising excess methanol, the reaction product methyl (meth)acrylate, and the reaction side product water.
The ester exchange reaction, which normally uses an excess molar amount of methyl (meth)acrylate with respect to an alcohol, consumes essentially all of the alcohol by the ester exchange reaction. Thus, the liquid reaction mixture after the reaction is a liquid mixture of the reaction side product methanol, unreacted methyl (meth)acrylate, and the reaction product ester. From among these components, the reaction product ester, which has a large boiling point difference from methanol and methyl (meth)acrylate, is relatively easy to separate, thereby providing a liquid mixture of the methanol and methyl (meth)acrylate free of the ester.
Several proposals have been made whereby an organic solvent capable of generating an azeotropic mixture with methanol is added to a mixture mainly comprising methyl (meth)acrylate and methanol, optionally containing water, followed by distilling to separate into methanol and methyl (meth)acrylate.
For example, a process is known in which a liquid mixture comprising methanol, water, and methyl (meth)acrylate is azeotropically distilled in the presence of an organic solvent and the entire amount of methanol is essentially stripped off the top of the distillation column and it is mostly free of water (see, e.g., U.S. Pat. No. 2,916,512; Japanese Laid-Open Publication 57-9740). The process, which strips off the methanol by azeotropic distillation, gives a distillate which contains the organic solvent. The process discloses that the distillate is fed to a decanter for separation into a layer mainly comprising the organic solvent and a layer mainly comprising methanol, and the organic solvent layer is returned to the distillation column. However, such a previously-known method, which efficiently distills and separates methanol from methyl (meth)acrylate, gives a distillate which still contains methyl (meth)acrylate, the distillate separating into two layers, with the lower layer mostly comprised of methanol mixed with methyl (meth)acrylate. Particularly, in the case of methyl acrylate, its low boiling point will give results in which the amount of contaminating methyl acrylate is more than negligible.
The esterification reaction recycles the separated methanol back to the reaction so that any contamination with methyl (meth)acrylate essentially poses no problem. However, the ester exchange reaction requires withdrawing out of the system the reaction by-product methanol, so that any methyl (meth)acrylate contained in the methanol gives rise to a recovery loss.
A proposal is given in Japanese Laid-Open Publication 57-9740 for reducing such recovery losses, which calls for controlling the type and amount of the organic solvent which forms an azeotropic mixture with methanol, and the number of plates for the methanol condensation section of the distillation column. The process, while exhibiting some effect, is still deficient in that the amount of methyl (meth)acrylate distillate cannot be substantially decreased. Japanese Laid-Open Publication 58-203940 teaches a process for recovering methanol, which comprises separating the distilled methanol and azeotropic solvent into two layers, feeding the upper layer mainly comprising the azeotropic solvent to the upper-most plate of the distillation column and feeding the lower layer, mostly comprised of methanol, into another distillation column (hereafter a second distillation column), recovering from the top of the second distillation column the azeotropic solvent that was dissolved in methanol, and recovering methanol from the bottom of the second distillation column. However, that process, which makes it possible to recover the azeotropic solvent, feeds a liquid mainly comprised of methanol to the second distillation column, where the contaminating methyl (meth)acrylate is not separated and contaminates the methanol which is recovered from the bottom of the second distillation column.
Such methanol before use in other applications must be freed of any impurities with boiling points higher than methanol by distillation so that any methyl (meth)acrylate contained in the methanol would be discarded with the impurities, creating a recovery loss and making the process deficient.